The mechanisms responsible for anti-arrhythmic protection during ischemia-reperfusion (IR) in exercised hearts are not fully understood. The purpose of this investigation was to examine whether the ATP-sensitive potassium channels in the mitochondria (mito K(ATP)) and sarcolemma (sarc K(ATP)) provide anti-arrhythmic protection in exercised hearts during IR. Male Sprague-Dawley rats were randomly assigned to cardioprotective treadmill exercise or sedentary conditions before IR (I = 20 min, R = 30 min) in vivo. Subsets of exercised animals received pharmacological inhibitors for mito K(ATP) (5-hydroxydecanoate) or sarc K(ATP) (HMR1098) before IR. Blinded analysis of digital ECG tracings revealed that mito K(ATP) inhibition blunted the anti-arrhythmic effects of exercise, while sarc K(ATP) inhibition did not. Endogenous antioxidant enzyme activities for total, CuZn, and Mn superoxide dismutase, catalase, and glutathione peroxidase from ischemic and perfused ventricular tissue were not mitigated by IR, although oxidative stress was elevated in sedentary and mito K(ATP)-inhibited hearts from exercised animals. These findings suggest that the mito K(ATP) channel provides anti-arrhythmic protection as part of exercise-mediated cardioprotection against IR. Furthermore, these data suggest that the observed anti-arrhythmic protection may be associated with preservation of redox balance in exercised hearts.